1. Field of the Invention
The present invention relates to the following subject matter: reflection-type masks for use in pattern exposure for projecting mask patterns thereof by reflecting exposure light, manufacturing methods therefor, exposure apparatuses using the reflection-type masks for use in pattern exposure, methods of manufacturing devices by using the exposure apparatuses, and semiconductor devices manufactured by the methods of manufacturing devices.
2. Description of the Related Art
In recent years, semiconductor integrated circuits having smaller features have been designed, and concomitant with this trend, exposure apparatuses for transferring circuit patterns on wafers are required to have the ability to transfer even finer circuit patterns. The abilities of exposure apparatuses largely depend on the wavelength of the exposure light, and hence, exposure apparatuses for exposing finer circuit patterns tend to use exposure light having shorter wavelengths. As exposure apparatuses having the ability to transfer a finer circuit pattern as described above, soft x-ray reduction projection exposure apparatuses have been investigated. The soft x-ray reduction projection exposure apparatus is an apparatus in which x-rays are generated from a light source, a reflection-type mask having a mask pattern formed therein is irradiated by the x-rays, and reductive projection is then performed onto a resist coated on a wafer by the x-rays reflected at the reflection-type mask.
As a reflection-type mask for use in soft x-ray reduction projection exposure apparatuses, a multilayer reflection film reflecting x-rays is typically provided on a substrate having a predetermined shape, which is formed by alternately depositing two types of materials having different refractive indexes. The combination of materials constituting a multilayer reflection film is changed in accordance with the wavelength of x-rays to be reflected. For example, for x-rays having a wavelength of approximately 13 nm, molybdenum (Mo) and silicon (Si) are used, and for x-rays having a wavelength of approximately 5 nm, chromium (Cr) and carbon (C) are typically used. In general, as the topmost surface of the multilayer reflection film, a material is selected having a refractive index largely differing from that in a vacuum or a processing atmosphere, and among the materials mentioned above, a Mo layer or a Cr layer may be used. Since a multilayer reflection film composed of Mo and Si most stably gives a high reflectance at present, the practical use thereof is hopefully expected.
The thicknesses of individual layers constituting the multilayer reflection film described above are determined by the wavelength of the incident x-rays, the incident angle thereof, constituent materials for the multilayer reflection film, and the like. When incident x-rays are perpendicular to the surface of the multilayer reflection film, the thickness of a pair of layers adjacent to each other is approximately one-half of the wavelength of the incident x-rays. In addition, in general, comparing the two layers mentioned above, a layer having a smaller absorption coefficient of x-rays is slightly thicker than the other layer having a larger absorption coefficient of x-rays. Accordingly, when x-rays having a wavelength of thirteen nm are used, the thickness of the Mo layer of the multilayer reflection film is slightly smaller than three nm, that is, the layer is extremely thin.
There are two major methods for manufacturing mask patterns. One method is that non-reflection areas, which do not reflect x-rays, are formed by covering a multilayer reflection film with a patterned absorption layer so as to form a mask pattern. The other method is that non-reflection areas are formed by removing or destroying parts of a multilayer reflection film so as to directly form a mask pattern in the multilayer reflection film.
FIG. 1 is a cross-sectional view of a conventional reflection-type mask for use in a soft x-ray reduction projection exposure apparatus. As shown in FIG. 1, a multilayer reflection film 9 is formed on a substrate 3 having a predetermined shape by alternately depositing Mo layers 1 and Si layers 5, which reflect x-rays. On the upper surface of the multilayer reflection film 9, an absorption layer 4 is formed for absorbing x-rays and is patterned so as to cover parts of the multilayer reflection film 9.
The absorption layer 4 is formed of a material having a large absorption coefficient of x-rays to be used. In general, a heavy element, such as gold (Au), tungsten (W), or tantalum (Ta), is used. The absorption coefficients of these materials mentioned above differ in accordance with the wavelength of the incident x-rays, and hence, a material constituting the absorption layer 4 is preferably selected based on the wavelength of x-rays to be used.
As a method for forming a mask pattern in the absorption layer 4, an electroplating method may be used. In this method, a resist layer (not shown) is first formed on the multilayer reflection film 9, and the resist layer is then patterned by using an electron beam lithographic method. Subsequently, the absorption layer 4 is formed by using an electroplating method, and a mask pattern is then formed in the absorption layer 4 by removing the resist layer by dry etching. In addition to the method using an electroplating method, a sputtering deposition method, an evaporation method, or the like method may be used, so as to form the absorption layer 4 on the multilayer reflection film 9.
An electroplating method has advantages in that the absorption layer 4 can be easily formed with less damage imposed on the multilayer reflection film 9. In xe2x80x9cSurface Technologyxe2x80x9d, Vol. 49, No. 8, (1998), pp. 47 to 51, it is reported that a superior absorption layer 4 composed of nickel (Ni) is obtained by an electroplating method.
When the absorption layer 54 is formed by an electroplating method, the topmost surface layer of the multilayer reflection film 9 must be composed of a conductive material. According to xe2x80x9cSurface Technologyxe2x80x9d, Vol. 49, No. 8, (1998), pp. 47 to 51, described above, when the topmost surface layer of the multilayer reflection film 9 is formed of a Mo layer 1, an absorption layer 4 composed of Ni is preferably formed, and when the topmost surface layer of the multilayer reflection film 9 is formed of a Si layer 5, Ni in a spherical shape is only formed, whereby no absorption layer 4 is practically obtained.
The reflectance of the absorption layer 4 depends on the thickness thereof. For example, when the thickness of the absorption layer 4, which consists of Au on the multilayer reflection film having a reflectance of 70%, is 30 nm, the x-ray reflectance thereof is 6%, and when the thickness of the absorption layer 4 is decreased by 10%, that is, when the thickness is decreased to 27 nm, the reflectance thereof is 7.7%. Accordingly, when the thickness of the absorption layer 4 is not uniform, the reflectance thereof varies within the absorption layer 4. The variation in reflectance in the absorption layer 4 results in errors in line widths of a circuit pattern on a wafer. Hence, the thickness of the absorption layer 4 must be uniform. The variation in reflectance of the absorption layer 4 can be controlled by sufficiently increasing the thickness of the absorption layer 4. However, incident x-rays to a reflection-type mask are not ideally perpendicular to the surface of the reflection-type mask and are inclined at an angle of some degrees thereto. In the case described above, when the thickness of the absorption layer is large, errors are generated in the circuit pattern on the wafer due to shadows cast by the thickness of the absorption layer 4. Accordingly, the thickness of the absorption layer 4 is preferably reduced to a minimum level as long as the absorption layer 4 serves as expected.
In addition, when the multilayer reflection film 9 is directly patterned, patterning may be performed by removing parts of the multilayer reflection film 9. However, in order to form non-reflection areas which do not reflect x-rays, since the reflectance of areas at which non-reflection areas are formed is merely decreased, patterning may be performed only by destroying parts of the multilayer structure of the multilayer reflection film 9. As a method of removing or destroying parts of the multilayer structure of the multilayer reflection film 9, a method of performing direct patterning by a converging ion beam (or focused ion beam) method or a method performing dry etching after a resist pattern is formed may be used.
As described above, in the steps of manufacturing reflection-type masks, there are many steps using charged particles, such as ions or electrons. In these steps, when the conductivity of a material to be processed is low, charge up occurs, that is, the material to be processed is electrified, and hence, the problems may arise in that the accuracy of a mask pattern to be formed is degraded or the like. Accordingly, in the steps of manufacturing reflection-type masks, the multilayer reflection film 9 must have sufficient conductivity.
The topmost surface layer of the multilayer reflection film 9 of the conventional reflection-type mask described above is formed of a conductive material; however, a layer in contact with the topmost surface layer is formed of a non-conductive material in many cases. As a result, the conductivity of the multilayer reflection film 9 is secured only by the topmost surface layer thereof.
The thickness of the topmost surface layer is approximately one-fourth of the wavelength of the x-rays, i.e., several nm, and hence, the conductivity of the topmost surface layer is only approximately one-tenth to one-to-several of the bulk value.
In addition, the reduction ratio of a reduction projection exposure apparatus is typically approximately one-fourth to one-fifth, and the size of the mask is 100 mm by 100 mm or more. In the steps of manufacturing a mask pattern, an electrode for securing the conductivity is provided outside the exposure area. Accordingly, the conductivity, which is required in the step of manufacturing the mask pattern, is not satisfactory in the most distant pattern portion from the electrode because of the extremely thin topmost surface layer of the multilayer reflection film.
In the case described above, when electron beam lithography is performed for the resist layer formed on the multilayer reflection film 9 for forming the absorption layer 4, it is difficult to remove charges accumulated in the resist layer when the conductivity of the multilayer reflection film 9 is low, which is located under the resist layer, whereby charge up occurs, and the pattern accuracy is degraded. In addition, when the multilayer reflection film 9 is directly patterned by a focused ion beam method, charge up occurs on the surface of the multilayer reflection film 9, and as a result, the pattern accuracy is degraded.
Furthermore, when the multilayer reflection film 9 cannot have sufficient conductivity even by an electroplating method, the thickness of the absorption layer 4 is gradually decreased in accordance with the distance from the electrode, and as a result, the thickness of the absorption layer 4 becomes uneven. In the case in which an electrode is provided at an edge of the multilayer reflection film 9, and the absorption layer 4 is formed by using a sulfite plating solution, an absorption layer 4 is formed having an uneven thickness. When the size of a mask is 100 by 100 mm, the thickness of the absorption layer 4 at the portion thereof, which is most distant from the electrode, is decreased by approximately 10%. When an absorption layer 4 having an uneven thickness can only be formed, the thickness of the absorption layer 4 cannot be decreased as described above, and consequently, the accuracies of line widths and a location of the circuit pattern formed on a wafer are degraded.
As described above, the conventional reflection-type mask for use in pattern exposure has the following two problems.
(1) When an electron beam lithographic method or a focused ion beam method is performed in the steps of manufacturing a mask pattern, since the multilayer reflection film has insufficient conductivity, charge up occurs in the multilayer reflection film, and as a result, the accuracy of the mask pattern is degraded. Consequently, the accuracies of line widths and a location of the circuit pattern formed on a wafer are degraded.
(2) When an electroplating method is performed in the step of forming an absorption layer, since the multilayer reflection film has insufficient conductivity, an absorption layer having an uneven thickness can only be formed, and hence, the reflectance of the absorption layer varies. As a result, the accuracies of line widths and a location of the circuit pattern formed on a wafer are degraded. In addition, since the absorption layer having an uneven thickness can only be formed, the thickness of the absorption layer cannot be decreased. As a result, the accuracies of line widths and a location of the circuit pattern formed on a wafer are degraded due to a shadowing effect of the x-rays.
Accordingly, it is an object of the present invention to provide a reflection-type mask for use in pattern exposure to improve the accuracies of line widths and a location of a circuit pattern formed on a wafer by eliminating bad influences caused by insufficient conduction in manufacturing steps. Accordingly, for example, charge up can be constrained, which is generated in a step of performing an electron beam lithographic method, a focused ion beam method, or the like, and an absorption layer having a uniform thickness can be formed in an electroplating method or the like.
In one aspect, the present invention provides a reflection-type mask for use in exposing a pattern onto a photosensitive material. The mask includes a reflection area, having a multi-layer film, for reflecting exposure light and a non-reflection area which does not reflect the exposure light, the reflection area and the non-reflection area forming a mask pattern. At least one layer of the multi-layer film consists of an impurity semiconductor.
In another aspect, the present invention provides a method of producing a reflection-type mask for use in exposing a pattern onto a photosensitive material, the mask comprising a reflection area, having a multi-layer film, for reflecting exposure light, and a non-reflection area which does not reflect the exposure light, wherein the reflection area and the non-reflection area form a mask pattern. The method includes forming the multi-layer film, such that one layer of the multi-layer film consists of an impurity semiconductor, and forming the non-reflection area.
In yet another aspect, the present invention provides an exposure apparatus that includes a light source for emitting exposure light and an illumination optical system for illuminating a reflection-type mask by the exposure light from the light source. The reflection-type mask includes a reflection area, having a multi-layer film, for reflecting exposure light and a non-reflection area which does not reflect the exposure light, the reflection area and the non-reflection area forming a mask pattern, and at least one layer of the multi-layer film consisting of an impurity semiconductor. A photosensitive material is exposed by the exposure light from the reflection-type mask, which is exposed by the illumination optical system.
In still another aspect, the present invention provides a method of manufacturing a device. The method includes illuminating a reflection-type mask by exposure light, the reflection-type mask comprising a reflection area, having a multi-layer film, for reflecting exposure light and a non-reflection area which does not reflect the exposure light, the reflection area and the non-reflection area forming a mask pattern, and at least one layer of the multi-layer film consisting of an impurity semiconductor, exposing a photosensitive material by the exposure light from the reflection-type mask and developing the photosensitive material exposed by the exposure light, such that a circuit pattern is produced by using the developed photosensitive material.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.